The invention relates to a method for a frequency converter, when the frequency converter controls a motor and operates torque-controlled partly or entirely in a field weakening region.
By using frequency converters, the rotation speed of a controlled motor can be increased considerably above the nominal frequency of the motor. Increasing the rotation speed above the nominal frequency requires typically a reduction in magnetization, whereby the output frequency increases above a field weakening point, i.e. the machine is said to be in a field weakening region.
Earlier, solutions based on direct torque control have tried to control a flux phasor in the field weakening region in such a manner that its head forms a circle in the spatial coordinates. At the same time they have tried to avoid using zero phasors in keeping the voltage in the motor poles as high as possible. A maximum voltage would be achieved, if the flux phasor formed a hexagon. Each angle of the hexagon would correspond to the directions of voltage vectors. With such a shape, it would be possible, in theory, to achieve a 20% increase in the torque in the field weakening region as compared with a circular flux trajectory. Eliminating the zero phasors has not, however, been successful with the earlier implementations and with a hexagonal flux, and thus, it has not been possible to utilize a maximum voltage.
In the earlier implementations, control is based on complex calculations that due to the complexity need to be done in a slow time domain, whereby the stability and dynamics of the control suffer at high rotation speeds.
It is an object of the present invention to provide a method that avoids the above-mentioned drawbacks and enables the control of a motor torque in a field weakening region more reliably than before. This object is achieved by a method of the invention that is characterized by comprising the steps of defining the direction of travel and sector of a stator flux vector in the motor, predicting a torque estimate produced by the output voltage vector of a frequency converter at the end of the voltage vector when a change of voltage vector occurs at the instant of prediction, comparing the predicted torque estimate with the frequency converter reference torque, and implementing the voltage vector change when the predicted torque estimate is smaller than the reference torque and the stator flux vector is moving in a positive direction of travel or when the torque estimate is greater than the reference torque and the stator flux vector is moving in a negative direction of travel.
The invention is based on the idea that the change caused by the voltage vector change to the torque can be predicted in a simple and reliable manner. In the method of the invention, the torque can be predicted in the fastest time domain of control, whereby the dynamics and stability of the control are ensured. In addition, by means of the method of the invention, it is possible to utilize in a field weakening region the entire voltage of the intermediate circuit of the frequency converter, since the control implemented by means of the method uses no zero-voltage vectors.